In the shifting of a stepped ratio transmission, clutches are engaged and disengaged to allow for power transfer through a plurality of different power paths. Typically, when a shift is performed, one clutch is disengaged (also known as an offgoing clutch) by decreasing an oil pressure on a piston of the clutch and another clutch is engaged (also known as an oncoming clutch) by increasing a pressure on a piston of the clutch. During an overlap shift, this process happens simultaneously in a coordinated manner. In a filling phase of a shift, the piston of the ongoing clutch is positioned adjacent a plurality of friction plates by regulating a pressure of the transmission fluid.
A positioning of the piston is performed by sending out a pulse width modulated (also known as PWM) signal from a controller to an electroproportional valve. In response to the signal, the electroproportional valve applies a pressure to a piston chamber of the clutch. Depending on a force created by this pressure, a position of the piston can be controlled. Typically, it is desired to position the piston adjacent a set of friction plates as fast as possible while making sure an engagement of the friction plates occurs in a smooth manner.
A pressure profile employed by the controller may be dependent on many variables, such as, but not limited to a plurality of mechanical characteristics of the clutch, a temperature of an automatic transmission fluid, a pressure within a fluid conduit, and an amount of air within the fluid conduit. Generally, these variables can be taken into account by scheduling the two parameters with which the pressure profile is parameterized.
A problem that remains however, is how to obtain a correct value for each of these parameters. The value should be specific for a transmission and even for an individual clutch. Currently, it is common practice for the filling parameters to be determined through a calibration process. The calibration process is performed following vehicle production and then the calibration process is repeated at fixed intervals based on a number of operating hours of the vehicle. Typically, the process takes place through the following steps. After a predetermined number of operating hours, the controller of the transmission indicates that a recalibration is advised. When the calibration process is started, the controller sends out a number of filling profiles with changing fill parameters to a valve of the transmission. This process is continued until adequate filling is achieved for the corresponding clutch. The timing of a drop in torque converter speed ratio is used as an indicator for a quality of the filling of the clutch. The drop is indicative of torque transfer through the clutch, which is a sign of the piston contacting the set of friction plates. The calibration process is then repeated for each of the remaining clutches.
While the calibration process described above is capable of determining the correct filling parameters, it does so only for fixed conditions. The calibration process is performed with a transmission that has been warmed up and a time between fillings is very short in duration. As a result, the parameters that are obtained are in fact only valid in conditions similar to those that were present during the calibration. During actual use of the transmission, artificial and approximate correction factors need to be applied to compensate for such a calibration. The correction factors are not in all cases a good representation of the characteristics of the actual system, which can lead to errors in the filling and consequently, poor shift quality.
Further, tolerances on the production process of the components of the transmission are partially responsible for the variability during the filling process. While generally accurate parameters can be obtained by performing a calibration following production, the system also changes as the friction plates wear, an automatic transmission fluid wears out, and a stiffness of a clutch spring deteriorates. The optimal values of these parameters change over time. The current typical calibration process which is used to solve these problems takes a considerable amount of time, and during the calibration process the vehicle cannot be used. As an amount of the time between recalibrations is not based on the actual condition of the transmission, but rather as a fixed number of operating hours, reducing a number of recalibrations is achieved by imposing limitations on the mechanical system. During production, tight tolerances are imposed on both components and assembly of the system. These tolerances, which increase a cost of the system, could be relaxed if a method were available to determine the correct parameters for the filling of a specific clutch and to keep them within acceptable bounds over a lifetime of the clutch.
Furthermore, only the usage of a single type of transmission fluid is recommended by the manufacturer, as the temperature or viscosity compensation factors are only valid for the recommended type of transmission fluid. Lack of versatility in this respect can increase ownership and maintenance costs of the vehicle.
Another problem with the current typical calibration process is that the transmission controller is not aware when a bad shift is performed as a result of unsuitable fill parameters. Even though the mechanical system might have changed considerably, the controller maintains use of the same parameters until the calibration process is initiated manually or the recommended number of operating hours between calibrations is reached.
It would be advantageous to develop a device and method for learning filling parameters for a clutch that eliminates a need for transmission calibrations, allows components having greater production tolerances to be used in the clutch, and allows a variety of transmission fluids to be used with the clutch.